The extreme load is in several cases the design driving factor in tower design. To improve the turbine power output it is necessary to increase the rotational speed of the turbine, both to reduce gear loads and improve aerodynamic efficiency. However, an increased rotational speed may result in higher extreme load.
Turbines today have a fixed maximum speed. At this speed the extreme loads are known and used to design the turbine. The turbine controller will constantly aim to hold this speed, especially at high wind speeds.
It turns out that the turbine controller has the biggest problem with extreme loads when the pitch angle is at optimum, which is the angle that optimizes the power extraction. This is due to the fact that the thrust force on the rotor is largest around this operation point.